Extract metadata from package files

To be able to run actions on package files we need to first extract their respective metadata.

This includes extracting .SRCINFO metadata, e.g. by using python-srcinfo and verifying it using pydantic models. The files list needs to be extracted from a package's .MTREE file (see man 5 mtree).

For the extraction of .BUILDINFO metadata (out of scope for this ticket) refer to #20 (closed).